US20060140260A1 - Power supply line communication modem and power supply line communication system - Google Patents

A modem for power-line communications (20A) comprises input/output terminals (71 a and 71 b) and input/output terminals (72 a to 72 d). The input/output terminals (71 a and 71 b) are connected to a power line and perform input and output of power-line communications signals. The input/output terminals (72 a to 72 d) perform input and output of data signals from and to a communications apparatus through a cable. The modem (20A) further comprises: a modem main body (73); a common mode filter (80) provided between the modem main body (73) and the input/output terminals (71 a and 71 b); and a common mode filter (90) provided between the modem main body (73) and the input/output terminals (72 a to 72 d).

TECHNICAL FIELD

• The present invention relates to a modem for power-line communications used in a power-line communications system that performs communications among a plurality of apparatuses through power lines as transmission lines of signals, and to the power-line communications system incorporating the modem for power-line communications.

BACKGROUND ART

• The needs for data communications in homes have been increasing to accomplish objectives such as the sharing of peripheral equipment of computers and the sharing of data including documents, freeze-frame pictures and moving pictures, and objectives such as games and the Internet. The demands for communications network systems therefore exist not only in offices but also in ordinary households. Communications schemes that can be chosen for constructing a communications network system in the households include the communications scheme utilizing radio, the communications scheme utilizing wires, and the power-line communications scheme utilizing power lines. Among these schemes, the power-line communications scheme has such benefits that no cost of installing wiring is required since existing power lines are used and that the appearance inside the house is not affected.

• However, the power-line communications scheme has a problem that communications interference could be caused by noise emerging from apparatuses connected to the power line. A variety of types of measures against noise have been therefore proposed for the power-line communications system.

• For example, the Published Unexamined Japanese Patent Application No. 2002-290288 and the Published Unexamined Japanese Patent Application No. 2002-290289 disclose a technique for providing a noise filter in the power-line communications system between a power line and an apparatus connected to the power line and developing noise.

• In the power line communications system, a communications apparatus that performs communications through the use of a power line is typically connected to the power line through a modem for power-line communications. In such a case, the communications apparatus is connected to the modem through a cable used for data communications such as a universal serial bus (USB). The communications apparatus herein described includes an information processing apparatus such as a computer and an electrical apparatus having a communications function, for example. The modem for power-line communications performs input and output of power-line communications signals from and to the power line and performs input and output of data communications signals from and to the cable used for data communications. The power-line communications signals and the data communications signals are both normal mode signals.

• A problem that will now be described occurs in the above-mentioned system including the modem for power-line communications. In this system, a stray capacitance is produced between the power line and the ground and/or between the cable used for data communications and the ground, for example. Then, a common mode current is fed through the stray capacitance to the ground when a normal mode signal passes through the power line or the cable. As a result, common mode noise emerges along the power line or the cable. According to prior art, it is impossible to effectively suppress common mode noise resulting from the stray capacitance in such a manner.

DISCLOSURE OF THE INVENTION

• It is an object of the invention to provide a modem for power-line communications for effectively suppressing common mode noise resulting from a stray capacitance between the ground and a power line or a cable connected to the modem, and to provide a power-line communications system incorporating the modem.

• A modem for power-line communications of the invention is provided between a power line and a communications apparatus performing communications through the use of the power line. The modem of the invention comprises: a first input/output connected to the power line and performing input and output of power-line communications signals from and to the power line; a second input/output connected to the communications apparatus and performing input and output of data signals from and to the communications apparatus; a modem main body that generates a power-line communications signal by modulating carrier waves based on a data signal received at the second input/output and outputs the power line communications signal to the first input/output, and that demodulates a data signal from a power-line communications signal received at the first input/output and outputs the data signal demodulated to the second input/output; and common mode filters one of which is provided between the modem main body and the first input/output and the other of which is provided between the modem main body and the second input/output.

• In the present patent application, the communications apparatus widely means apparatuses performing communications through the use of power lines and includes information processing apparatuses such as computers and electrical apparatuses having a communications function.

• In the modem of the invention, passing of common mode currents is suppressed at both of the first input/output and the second input/output by the common mode filters one of which is provided between the modem main body and the first input/output and the other of which is provided between the modem main body and the second input/output. It is thereby possible to suppress the emergence of common mode noise.

• In the modem of the invention, the absolute value of impedance of each of the common mode filters at a frequency of 2.0 MHz may be 160 ohms or greater.

• A power-line communication system of the invention comprises: a power line; a communications apparatus performing communications through the use of the power line; and the modem of the invention provided between the communications apparatus and the power line.

• Other and further objects, features and advantages of the invention will appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1

  is a schematic diagram illustrating an example of configuration of a modem for power-line communications of an embodiment of the invention.

• FIG. 2

  is a block diagram illustrating an example of configuration of a power-line communications system in which the modem for power-line communications of the embodiment of the invention is employed.

• FIG. 3

  is a view for describing a cause of common mode noise in the power-line communications system of

  FIG. 2

  .

• FIG. 4

  is a schematic diagram illustrating an example of configuration of a power source of a communications apparatus of

  FIG. 2

  .

• FIG. 5

  is a schematic diagram for describing an impedance of a common mode filter of

  FIG. 1

  .

• FIG. 6

  is a schematic diagram for describing an impedance of the common mode filter of

  FIG. 1

  .

• FIG. 7

  is a schematic diagram for describing an impedance of the common mode filter of

  FIG. 1

  .

BEST MODE FOR CARRYING OUT THE INVENTION

• A preferred embodiment of the invention will now be described in detail with reference to the accompanying drawings. Reference is now made to

  FIG. 2

  to describe an example of configuration of a power-line communications system in which a modem for power-line communications of the embodiment of the invention is employed. The power-line communications system of

  FIG. 2

  comprises:

  power lines

  1A to 1G; two

  communications apparatuses

  10A and 10B performing communications through the use of the

  power lines

  1A to 1G; and

  modems

  20A and 20B for power-line communications of the embodiment. The

  communications apparatuses

  10A and 10B may be information processing apparatuses such as computers or electrical apparatuses having a communications function. Although each of the

  power lines

  1A to 1G is indicated with a single line in

  FIG. 2

  , each of them actually includes a plurality of conductor lines.

• The

  power line

  1B is connected to the

  power line

  1A. The

  power line

  1C has an end connected to the

  power line

  1B and the other end connected to the

  modem

  20A. The

  power line

  1D has an end connected to the

  power line

  1B and the other end connected to the

  modem

  20B.

• The

  power line

  1E is connected to the

  power line

  1A. The

  power line

  1F has an end connected to the

  power line

  1E and the other end connected to the

  communications apparatus

  10A. The

  power line

  1G has an end connected to the

  power line

  1E and the other end connected to the

  communications apparatus

  10B.

• The

  communications apparatus

  10A incorporates a

  power source

  11A and a

  communications interface

  12A. Similarly, the

  communications apparatus

  10B incorporates a

  power source

  11B and a

  communications interface

  12B. The

  power sources

  11A and 11B are connected to the

  power lines

  1F and 1G, respectively, and receive power supply through the

  power lines

  1F and 1G. The

  interfaces

  12A and 12B perform transmission and reception of data signals to and from the

  modems

  20A and 20B, respectively.

• The

  modem

  20A has a first input/

  output

  21A and a second input/

  output

  22A. The first input/

  output

  21A performs input and output of power-line communications signals from and to the

  power line

  1C. The second input/

  output

  22A is connected to the

  interface

  12A of the

  communications apparatus

  10A through a

  cable

  3A for data communications such as a USB. The second input/

  output

  22A performs input and output of data signals from and to the

  interface

  12A through the

  cable

  3A.

• The

  modem

  20A generates a power-line communications signal by modulating carrier waves based on a data signal received at the second input/

  output

  22A, and outputs the power-line communications signal from the first input/

  output

  21A. In addition, the

  modem

  20A demodulates a data signal from a power-line communications signal received at the first input/

  output

  21A, and outputs from the second input/

  output

  22A the data signal demodulated.

• Similarly, the

  modem

  20B has a first input/

  output

  21B and a second input/

  output

  22B. The first input/

  output

  21B performs input and output of power-line communications signals from and to the

  power line

  1D. The second input/

  output

  22B is connected to the

  interface

  12B of the

  communications apparatus

  10B through a

  cable

  3B for data communications such as a USB. The second input/

  output

  22B performs input and output of data signals from and to the

  interface

  12B through the

  cable

  3B.

• The

  modem

  20B generates a power-line communications signal by modulating carrier waves based on a data signal received at the second input/

  output

  22B, and outputs the power-line communications signal from the first input/

  output

  21B. In addition, the

  modem

  20B demodulates a data signal from a power-line communications signal received at the first input/

  output

  21B, and outputs from the second input/

  output

  22B the data signal demodulated.

• In

  FIG. 2

  , arrows with

  numerals

  4A, 4B and 4C indicate power-line communications signals. Arrows with

  numerals

  5A and 5B indicate data signals. The power-line communications signals and the data signals are both normal mode signals.

• In the power-line communications system of

  FIG. 2

  , the

  communications apparatuses

  10A and 10B perform communications with each other through the

  cables

  3A and 3B, the

  modems

  20A and 20B, and the

  power lines

  1B, 1C and 1D.

• Reference is now made to

  FIG. 3

  to describe a cause of common mode noise in the power-line communications system of

  FIG. 2

  .

  FIG. 3

  illustrates the

  modem

  20A, the

  communications apparatus

  10A, the

  cable

  3A, and the

  power lines

  1A, 1B, 1C, 1E and 1F among the components of the power line communications system of

  FIG. 2

  , and illustrates a ground 2. Stray capacitance develops at various locations in the power-line communications system.

  FIG. 3

  illustrates: a stray capacitance 31 between the

  power line

  1C and the ground 2; a stray capacitance 32 between the enclosure of the

  modem

  20A and the ground 2; a stray capacitance 33 between the

  cable

  3A and the ground 2; a stray capacitance 34 between the enclosure of the

  communications apparatus

  10A and the ground 2; a stray capacitance 35 between the

  power line

  1F and the ground 2; and a stray capacitance 36 between the

  power line

  1A and the ground 2. The enclosure of the

  modem

  20A is used as a frame ground of the

  modem

  20A.

• In the system of

  FIG. 3

  , when a normal mode signal passes through the

  power line

  1C, a common mode current is fed to the ground 2 through the stray capacitance 31. The common mode current is fed through a closed path including the inputs/

  outputs

  21A and 22A and the other stray capacitances in addition to the stray capacitance 31. When a normal mode signal passes through the

  cable

  3A, a common mode current is fed to the ground 2 through the stray capacitance 33. The common mode current is fed through a closed path including the inputs/

  outputs

  21A and 22A and the other stray capacitances in addition to the stray capacitance 33. In

  FIG. 3

  , the arrows with numerals 41 to 52 indicate the above-mentioned common mode currents. When such a common mode current is fed, a common mode current 55 is fed to the

  power line

  1C, and a common mode current 56 is fed to the

  cable

  3A. These common mode currents 55 and 56 cause common mode noise.

• A common mode current passes through the

  communications apparatus

  10A in some cases. This will now be described, referring to

  FIG. 4

  .

  FIG. 4

  is a schematic diagram illustrating an example of configuration of the

  power source

  11A of the

  communications apparatus

  10A. In this example, the

  power source

  11A comprises: a primary-side circuit 61 connected to the

  power line

  1F; a secondary-side circuit 62 supplying power to each part of the

  communications apparatus

  10A; and a transformer 63 connecting the primary-side circuit 61 to the secondary-side circuit 62. The transformer 63 has: a primary winding 63 a connected to the primary-side circuit 61; a secondary winding 63 b connected to the secondary-side circuit 62; and a magnetic core 63 c coupling the primary winding 63 a to the secondary winding 63 b. In the

  power source

  11A, for example, a stray capacitance 64 is produced between the primary winding 63 a and the secondary winding 63 b, and a stray capacitance 65 is produced between the primary-side circuit 61 and the secondary-side circuit 62. A common mode current passes through the

  power source

  11A via the stray capacitances 64 and 65, and thereby passes through the

  communications apparatus

  10A.

• To suppress common mode noise in the system of

  FIG. 3

  , it is required to suppress the common mode currents 55 and 56. In the

  modem

  20A of the embodiment, passing of common mode currents is suppressed through the use of a common mode filter at each of the first input/

  output

  21A and the second input/

  output

  22A, which will be described in detail later.

• Even if a common mode filter is provided at a location other than the inputs/

  outputs

  21A and 22A, it is impossible to sufficiently suppress common mode noise. For example, if a common mode filter is provided between the

  cable

  3A and the

  interface

  12A of the

  communications apparatus

  10A, a common mode current is fed through the stray capacitance 33 between the

  cable

  3A and the ground 2. The common mode current flows through the closed path including the stray capacitances 31 and 33 and the inputs/

  outputs

  21A and 22A, for example. As a result, common mode noise emerges.

• It is impossible to sufficiently suppress common mode noise by providing a common mode filter at only one of the first input/

  output

  21A and the second input/

  output

  22A. For example, if a common mode filter is provided at the first input/

  output

  21A, a common mode current is fed through a closed path including: two or more of the stray capacitances 32 to 36; the enclosure of the

  modem

  20A; and the input/

  output

  22A. As a result, common mode noise emerges. If a common mode filter is provided at the second input/

  output

  22A, a common mode current is fed through a closed path including: at least one of the stray capacitances 31 and 36; the stray capacitance 32; the enclosure of the

  modem

  20A; and the input/

  output

  21A. As a result, common mode noise emerges.

• If passing of the common mode current is blocked at both of the first input/

  output

  21A and the second input/

  output

  22A, it is possible to block the path of the common mode current. Therefore, it is possible to effectively suppress common mode noise by suppressing passing of the common mode current by using the common mode filters at both of the first input/

  output

  21A and the second input/

  output

  22A, as disclosed in the embodiment.

• Reference is now made to

  FIG. 1

  to describe an example of configuration of the

  modem

  20A for power-line communications of the embodiment. The

  modem

  20B of

  FIG. 2

  has a configuration the same as that of the

  modem

  20A.

• The

  modem

  20A of

  FIG. 1

  comprises input/

  output terminals

  71 a and 71 b and input/

  output terminals

  72 a to 72 d. The input/

  output terminals

  71 a and 71 b are connected to the

  power line

  1C and perform input and output of power-line communications signals from and to the

  power line

  1C. The input/

  output terminals

  71 a and 71 b correspond to the first input/

  output

  21A of

  FIG. 2

  . The input/

  output terminals

  72 a to 72 d are connected to the

  cable

  3A and perform input and output of data signals from and to the

  communications apparatus

  10A connected to the

  terminals

  72 a to 72 d through the

  cable

  3A. The input/

  output terminals

  72 a to 72 d correspond to the second input/

  output

  22A of

  FIG. 2

  .

• The

  modem

  20A further comprises: a modem

  main body

  73; a

  common mode filter

  80 provided between the modem

  main body

  73 and the input/

  output terminals

  71 a and 71 b; and a

  common mode filter

  90 provided between the modem

  main body

  73 and the input/

  output terminals

  72 a to 72 d.

• The modem

  main body

  73 incorporates: a

  communications control circuit

  74; a

  coupler circuit

  75 provided between the

  control circuit

  74 and the

  common mode filter

  80; and a

  communications interface

  76 provided between the

  control circuit

  74 and the

  common mode filter

  90.

• The

  coupler circuit

  75 blocks passing of electric power and allows power-line communications signals to pass. The

  interface

  76 performs transmission and reception of data signals to and from the

  interface

  12A of the

  communications apparatus

  10A through the

  cable

  3A.

• The

  control circuit

  74 generates a power-line communications signal by modulating carrier waves based on a data signal received from the

  interface

  76, and outputs the power-line communications signal to the

  coupler circuit

  75. In addition, the

  control circuit

  74 demodulates a data signal from a power-line communications signal received from the

  coupler circuit

  75, and outputs the data signal demodulated to the

  interface

  76.

• As thus described, the modem

  main body

  73 generates a power-line communications signal by modulating carrier waves based on data signals received at the input/

  output terminals

  72 a to 72 d, and outputs the power-line communications signal to the input/

  output terminals

  71 a and 71 b. In addition, the modem

  main body

  73 demodulates a data signal from a power-line communications signal received at the input/

  output terminals

  71 a and 71 b, and outputs the data signal demodulated to the input/

  output terminals

  72 a to 72 d.

• The

  common mode filter

  80 incorporates two

  windings

  81 a and 81 b and a

  magnetic core

  82 coupling the two

  windings

  81 a and 81 b to each other. The

  windings

  81 a and 81 b have ends connected to the input/

  output terminals

  71 a and 71 b, respectively, and the other ends connected to the

  coupler circuit

  75. The

  windings

  81 a and 81 b are wound around the

  core

  82 in such directions that, when magnetic fluxes are induced in the

  core

  82 by currents flowing through the

  windings

  81 a and 81 b when a common mode current is fed to the

  windings

  81 a and 81 b, the directions of these fluxes are identical. The

  windings

  81 a and 81 b thereby suppress common mode noise and allow normal mode signals to pass.

• The

  common mode filter

  90 incorporates four

  windings

  91 a to 91 d and a

  magnetic core

  92 coupling the four

  windings

  91 a to 91 d to one another. The

  windings

  91 a to 91 d have ends connected to the input/

  output terminals

  72 a to 72 d, respectively, and the other ends connected to the

  interface

  76. The

  windings

  91 a to 91 d are wound around the

  core

  92 in such directions that, when magnetic fluxes are induced in the

  core

  92 by currents flowing through the

  windings

  91 a to 91 d when a common mode current is fed to the

  windings

  91 a to 91 d, the directions of these fluxes are identical. The

  windings

  91 a to 91 d thereby suppress common mode noise and allow normal mode signals to pass.

• Reference is now made to

  FIG. 5

  to

  FIG. 7

  to describe the impedance of the

  common mode filters

  80 and 90.

  FIG. 5

  is a schematic diagram illustrating a

  common mode filter

  100 and

  conductor lines

  104 a and 104 b connected thereto. The

  common mode filter

  100 represents the

  common mode filters

  80 and 90. The conductor lines 104 a and 104 b represent the

  power line

  1C and the

  cable

  3A.

• The

  common mode filter

  100 incorporates two

  windings

  101 a and 101 b and a

  magnetic core

  102 coupling the two

  windings

  101 a and 101 b to each other. The

  conductor line

  104 a is connected to the winding 101 a through a terminal 103 a. The

  conductor line

  104 b is connected to the winding 101 b through a terminal 103 b. Here, it is assumed that

  stray capacitances

  105 a and 105 b are created between the ground and the

  conductor lines

  104 a and 104 b, respectively.

• FIG. 6

  is a schematic diagram illustrating the circuit of

  FIG. 5

  in a simplified manner. In

  FIG. 6

  , the

  common mode filter

  100 is connected to a

  conductor line

  104 through a terminal 103. A

  stray capacitance

  105 is created between the

  conductor line

  104 and the ground. The

  stray capacitance

  105 is a combination of the

  stray capacitances

  105 a and 105 b of

  FIG. 5

  .

• FIG. 7

  is an equivalent circuit of the circuit shown in

  FIG. 6

  . As shown in

  FIG. 7

  , the absolute value of the impedance of the

  common mode filter

  100 is Z₁, and the absolute value of the impedance of the

  stray capacitance

  105 is Z₂. Here, a case is considered in which a high

  frequency voltage source

  110 applies a high frequency voltage V_A to a side of the

  common mode filter

  100 opposite to the terminal 103. In this case, the voltage V_N at a

  node

  106 between the

  conductor line

  104 and the

  stray capacitance

  105 is given by the following equation (1).
  V _N =V _A ×{Z ₂/(Z ₁ +Z ₂)}  (1)

• The high

  frequency voltage source

  110 corresponds to the modem

  main body

  73 of

  FIG. 1

  . The frequency of the high frequency voltage V_A corresponds to the frequency of a normal mode signal. The voltage V_N corresponds to the voltage of common mode noise emerging along the

  conductor line

  104. As the equation (1) indicates, the greater the absolute value Z₁ of the impedance of the

  common mode filter

  100, the smaller is the voltage of common mode noise. To suppress common mode noise, it is preferred that the absolute value Z₁ of the impedance of the

  common mode filter

  100 is equal to or greater than the absolute value Z₂ of the impedance of the

  stray capacitance

  105.

• The absolute value Z₂ of the impedance of the

  stray capacitance

  105 at the frequency of a normal mode signal is given by the following equation (2) where the angular frequency of the normal mode signal is ω and the capacitance of the

  stray capacitance

  105 is Cs.
  Z ₂=1/ωCs  (2)

• Consequently, the absolute value Z₁ of the impedance of the

  common mode filter

  100 is made equal to or greater than the absolute value Z₂ of the impedance of the

  stray capacitance

  105 as long as the following equation (3) holds.
  Z ₁≧1/ωCs  (3)

• The greater the capacitance Cs of the

  stray capacitance

  105, the smaller is the absolute value Z₂ of the impedance. As a result, the current flowing through the

  stray capacitance

  105 increases and the common mode noise becomes a problem. The capacitance of stray capacitance created along the

  cable

  3A is typically around 100 pF. Therefore, it is assumed here that the upper limit of the capacitance Cs of the

  stray capacitance

  105 is 500 pF. In addition, it is assumed that the lower limit of frequency band used in power-line communications is 2.0 MHz and this is the lower limit of frequency band of normal mode signals. In this case, as denoted by the equation (3), it is preferred that the absolute value Z₁ of the impedance of the

  common mode filter

  100 at the frequency of 2.0 MHz is equal to or greater than 160 ohms.

• The left side of the equation (3) is proportional to the angular frequency ω while the right side is inversely proportional to the angular frequency ω. Therefore, the equation (3) holds in the entire frequency band if the equation (3) holds at the frequency of the lower limit of the frequency band used for power-line communications unless there is no reduction in characteristics of the

  common mode filter

  100.

• According to the

  modem

  20A for power-line communications of the embodiment as thus described, the

  common mode filter

  80 is provided between the modem

  main body

  73 and the input/

  output terminals

  71 a and 71 b. In addition, the

  common mode filter

  90 is provided between the modem

  main body

  73 and the input/

  output terminals

  72 a to 72 d. As a result, according to the embodiment, it is possible to suppress passing of common mode currents at both of the first input/

  output

  21A connected to the

  power line

  1C and the second input/

  output

  22A connected to the

  cable

  3A. It is thereby possible to effectively suppress common mode noise resulting from the stray capacitance between the ground and the power line or cable connected to the

  modem

  20A in the power-line communications system.

• The present invention is not limited to the foregoing embodiment but may be practiced in still other ways. For example, the modem for power-line communications of the invention may be provided in a single enclosure together with a communications apparatus.

• According to the modem for power-line communications and the power-line communications system of the invention as thus described, the common mode filter is provided at each of the location between the modem main body and the first input/output connected to the power line and the location between the modem main body and the second input/output connected to the communications apparatus. As a result, according to the invention, it is possible to effectively suppress common mode noise resulting from the stray capacitance between the ground and the power line or cable connected to the modem.

• Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.